Ajit Pai, chairman of the Federal Communications Commission, said both companies had promised his agency they would offer a next-generation 5G network to 99 percent of Americans within six years of completing the deal while also expanding access to mobile broadband.

"The construction of this network and the delivery of such high-speed wireless services to the vast majority of Americans would substantially benefit consumers and our country as a whole," Pai said in a statement.

Failure to make good on their pledges could result in billions in payments to the US Treasury, creating an incentive for the companies to meet their obligations on time, according to Pai.

The commission is due to consider the merger proposal next month but still requires approval from the Justice Department.

T-Mobile and Sprint are respectively the third- and fourth-largest wireless carries in the US in terms of number of customers.

The two firms had previously called off merger talks after failing to clinch mutually agreeable terms.

5G, or fifth-generation, wireless communications networks would enable services such as remote surgery or driverless cars and allow customers to experience video and virtual reality with greater ease.

Together, T-Mobile and Sprint have about 131 million subscribers, virtually matching second-ranked AT&T and posing stiff competition to market leader Verizon Communications.